Process oils are generally understood to be hydrocarbon mixtures that boil in the lubricating oil range, but are usually not used as, and also not in combination with, lubricating oils. Process oils are obtained in the refining of petroleum. The crude oil is subjected to an atmospheric distillation, separating all products that boil up to about 350° C. at normal pressure. A mixture of bitumens, asphaltenes, waxes, and heavy oils is obtained as residue after the distillation. The heavy oils are processed further to various products, which in addition to lubricating oils also include the process oils, which are chiefly used as plasticizers.
The process oils differ in each case according to their content of aromatic carbon atoms (CA), naphthenic carbon atoms (CN), and paraffinic carbon atoms (CP), measured in accordance with ASTM D 2140. Aromatic process oils in some case have undesirably high amounts of polycyclic aromatics (PCAs). Polycyclic aromatics are understood to be compounds with more than two condensed aromatic nuclei. Since the polycyclic aromatics, such as benzo[a]pyrene, are suspected of being carcinogenic, even in the past process oils with a high PCA content have been used only to a limited extent.
According to European Guideline 769/76, augmented by Guideline 69/2005 of Nov. 16, 2005, the use of process oils is now only allowed if these process oils have a content of polycyclic aromatics that is less than 3 wt %, measured by the method of IP 346.
Besides the process oils that have a high aromatic content, there is another group of frequently used process oils, the naphthenic oils. Naphthenic oils are characterized by a high content of cycloalkanes, but can also have a higher content of aromatic hydrocarbon compounds than paraffinic oils. Correspondingly, naphthenic oils exhibit better solvent properties with respect to rubber than paraffinic oils and can be processed better. Naphthenic process oils for medicinal use usually contain no or only a small content of aromatics.
The corresponding process oils that still contain more than 3 wt % polycyclic aromatics in accordance with IP 346 must in the future either be disposed of as hazardous waste or, if need be, added to heating oil, where this has the disadvantage that higher sulfur emissions arise when the heating oil is burned. Because of the changed legislation such products in the future therefore may no longer be added to heating oil, in order to reduce sulfur emissions. If need be, combustion in plants with special filters will still be permitted. Moreover, extraction residues with a high content of polycyclic aromatics are formed in the production of process oils with a low content of PCAs, for example the process oils TDAE and MES. These extraction residues in the future also must be disposed of or added to heating oil.
One possibility for obtaining process oils with a low content of polycyclic aromatics is to reextract a primary extract that is obtained by extraction of a lubricating oil distillate that derives from mineral oil. One such process is described in EP 0 417 980 B1. The process oil that is obtained in this case, for example TDAE, has a polycyclic aromatic content that is less than 3 wt % in accordance with IP 346. However, a disadvantage of this process is that a product with a high concentration of polycyclic aromatics, from up to 15 and even over 20 wt %, is obtained as secondary extract, i.e., as the “bottom phase,” which is taken from the second extraction column.